Singlet L-a and L-b Bands for N-Acenes (N = 2-7): A CASSCF/CASPT2 Study

0479628 - ÚOCHB 2018 RIV US eng J - Journal Article
Bettanin, F. - Ferrao, L. F. A. - Pinheiro Jr., M. - Aquino, A. J. A. - Lischka, H. - Machado, F. B. C. - Nachtigallová, Dana
Singlet L-a and L-b Bands for N-Acenes (N = 2-7): A CASSCF/CASPT2 Study.
Journal of Chemical Theory and Computation. Roč. 13, č. 9 (2017), s. 4297-4306. ISSN 1549-9618. E-ISSN 1549-9626
R&D Projects: GA ČR(CZ) GA16-16959S
Institutional support: RVO:61388963
Keywords : polycyclic aromatic hydrocarbons * density functional theory * theory/multireference configuration interaction
OECD category: Physical chemistry
Impact factor: 5.399, year: 2017

In this work CASPT2 calculations of polyacenes (from naphthalene to heptacene) were performed to find a methodology suitable for calculations of the absorption spectra, in particular of the La (B-2u state) and L-b (B-3u state) bands, of more extended systems. The effect of the extension of the active space and of freezing a orbitals was investigated. The MCSCF excitation energy of the B-2u state is not sensitive to the size of the active space used. However, the CASPT2 results depend strongly on the amount of a orbitals frozen reflecting the ionic character of the B-2u state. On the other hand, the excitation energies of the B-3u state are much more sensitive to the size of the active space used in the calculations reflecting its multiconfigurational character. We found a good agreement with experimental data for both bands by including 14 electrons in 14 pi orbitals in the active space followed by the CASPT2(14,14) perturbation scheme in which both sigma and pi orbitals are included.
Permanent Link: http://hdl.handle.net/11104/0275606